There has been known a fluorescent lamp such as a low pressure mercury vapor discharge lamp provided with a U-shaped bulb formed with bending or connecting processes, from a publication such as Japanese Unexamined Patent Publication No. 155675/1979 (FIG. 1) or Japanese Unexamined Patent Publication No. 133744/1980 (FIG. 2). The fluorescent lamp having such construction is advantageous because a lamp is made in a compact form. However, the U-shaped bulb 1 as shown in the figure has a space of gap 31 between both legs 1a, 1b. Therefore, in manufacturing steps, when the legs 1a, 1b of the bulb 1 are gripped from their outer sides so that a force is applied in its central direction, a connecting part 2a between the both legs 1a, 1b is apt to cause breakage. Accordingly, a device A is generally used to grip the both legs 1a, 1b of the bulb 1 in a plan passing through the legs 1a, 1b, as shown in FIG. 3 which is disclosed in Japanese Unexamined Patent Publication No. 124928/1980. On the other hand, the fluorescent lamp in a compact form which is substituted for an incandescent lamp generally has the legs 1a, 1b of the bulb 1 of about 100-150 mm in height, on account of which the bulb has to be supported in only one direction in the manufacturing steps. This means that it is necessary to prepare the device A used for various manufacturing steps at a high accuracy thereby to cause problems of complicated maintenance and inspection.
Further, there has been proposed a fluorescent lamp in which a U-shaped bulb 1 is further bent into two parts (hereinbelow referred to as a double U-shaped bulb) aiming at further compactness of the lamp. FIG. 4 shows a lamp, as an example, disclosed in Japanese Unexamined Patent Publication No. 108162/1980. In this case, two U-shaped bulbs 1, 1 are arranged so that there remain spaces of gap 31, 32, 33 and 34 between all adjoining legs among the legs 1a, 1b, 1c and 1d. Therefore, when the two U-shaped bulbs have to be supported by a supporting device A without causing breakage of the connecting parts 2a, 2b, the structure of the device A must be complicated in comparison with the device for a single U-shaped bulb 1, this prohibiting a large scale production. The same condition applies to a fluorescent lamp of a double tube structure in which a U-shaped bulb is placed inside an outer bulb.